1. Field of the Invention
The present invention relates to a so-called tandem-type color image forming apparatus.
2. Description of Related Art
So-called tandem-type color image forming apparatuses, in which multiple image forming units housing toners of different colors are aligned along an intermediate transfer belt, are known in the conventional art. In an image forming apparatus of this type, images of different colors formed by the respective image forming units are transferred onto the intermediate transfer belt in a primary transfer operation such that they overlap each other, and these overlapping images undergo a secondary transfer operation as one unit from the intermediate transfer belt to a sheet of paper. The sheet of paper is heated such that the images bond thereto while it passes through the fusing unit, and is ejected to the eject tray.
The color image forming apparatus is also equipped with multiple color modes, including full color mode in which image formation is performed u sing color toners, and monochrome mode with which image formation is performed using black toner only. When the full color mode is activated, the color image forming apparatus forms color images by placing the image forming units of all colors in contact with the intermediate transfer belt. When the monochrome mode is activated, it performs formation of monochrome images by placing only the black image forming unit in contact with the intermediate transfer belt.
In the tandem-type color image forming apparatus described above, the status of contact between the intermediate transfer belt and each image forming unit must therefore be altered depending on the activated color mode. Where a color image and a monochrome image are printed on a continuous basis, if the next image forming operation is begun while the contact status between the intermediate transfer belt and the image forming units is still being changed or immediately after such change is conducted, poor image quality may result due to the vibration or snaking of the intermediate transfer belt.
However, if the next image forming operation is begun only after the vibration or snaking of the intermediate transfer belt caused by the mode switching operation has been completely stopped, productivity is greatly reduced.